The O-aryl esters of N-substituted carbamic acids include many compounds that have been found to be useful in agricultural applications. One such O-aryl carbamate is p-chlorophenyl-N-methyl carbamate. The aforesaid esters are prepared by the reaction of the corresponding aromatic alcohol, e.g., p-chlorophenol, with monoisocyanate, e.g., methyl isocyanate. The reaction between the aromatic alcohol and monoisocyanate can be accelerated by use of a tertiary amine, such as triethylamine or pyridine. The reaction can be conducted in the presence of an inert organic solvent; however, it can also be conducted in the absence of an extraneous inert organic solvent, as described in Canadian Pat. No. 984,405.
When conducted in the substantial absence of an extraneous solvent and with pyridine as the catalyst, the O-aryl carbamate product typically contains undesirable levels of allophanate by-product and the unreacted aromatic alcohol. Because the aforesaid O-aryl carbamates are used as agricultural chemicals, the levels of allophanate by-product and unreacted alcohol reactant therein is important for the reason that too high a level of such compounds can prevent governmental clearance of the use of the product in commerce.
It has now been discovered that O-aryl esters of substituted carbamic acids containing less than 0.8 weight percent allophanate by-product and 0.5 percent unreacted aromatic alcohol reactant can be prepared in the absence of an extraneous organic solvent by reacting the aromatic alcohol and monoisocyanate in the presence of a catalytic amount of the combination of a 4-aminopyridine catalyst, e.g., a 4-dialkylaminopyridine catalyst such as 4-dimethylaminopyridine, and a pyridine catalyst selected from pyridine, methylpyridines and dimethylpyridines. The aforesaid catalytic process provides numerous benefits; namely, (1) it proceeds readily to completion without significant refluxing of the isocyanate reactant and is, therefore, readily controlled; (2) it produces a substantially pure O-aryl carbamate product, which is satisfactory for commercial applications; (3) it involves less manufacturing cost than when a solvent is utilized; and (4) it permits the product to be maintained at reaction temperature for an extended time period without significantly affecting the level of by-product allophanate.